1. Field of the Invention
The present invention relates to a method of detection of output fluctuation in a multi-cylinder engine.
2. Description of the Related Art
Known in the art is an internal combustion engine which finds a first angular velocity of the crankshaft in the time required for the crankshaft to rotate from 30.degree. to 60.degree. after top dead center of the compression stroke from this period, finds a second angular velocity of the crankshaft in the time required for the crankshaft to rotate from 90.degree. to 120.degree. after top dead center of the compression stroke from this time, finds the torque generated by a cylinder from the square of the first angular velocity and the square of the second angular velocity, and calculates the amount of torque fluctuation from the amount of fluctuation of the generated torque (see Japanese Examined Patent Publication (Kokoku) No. 7-33809).
That is, when combustion is performed in a cylinder, the combustion pressure causes the angular velocity of the crankshaft to rise from a first angular velocity .omega.a to a second angular velocity .omega.b. At this time, if the moment of inertia of rotation of the engine is I, the combustion pressure causes the kinetic energy to rise from (1/2).multidot.I.omega.a.sup.2 to (1/2).multidot.I.omega.b.sup.2. Roughly speaking, the amount of rise of the kinetic energy (1/2).multidot.I.multidot.(.omega.b.sup.2 -.omega.a.sup.2) causes a torque to be generated, so the generated torque becomes proportional to (.omega.b.sup.2 -.omega.a.sup.2). Therefore, the generated torque is found from the difference between the square of the first angular velocity .omega.a and the square of the second angular velocity .omega.b and, therefore, in the above-mentioned internal combustion engine, the amount of torque fluctuation is calculated from the thus found generated torque.
However, if the generated torque is calculated based on the angular velocity .omega.a and .omega.b in this way, when for example the engine drive system experiences torsional vibration, the generated torque calculated based on the angular velocities .omega.a and .omega.b will no longer express the true generated torque. That is, when the engine drive system does not experience a torsional vibration, the second angular velocity .omega.b increases from the first angular velocity .omega.a by exactly the amount of increase of the angular velocity caused by the combustion pressure. As opposed to this, when the engine drive system experiences a torsional vibration, the second angular velocity .omega.b will include in addition to the amount of increase of the angular velocity caused by the combustion pressure the amount of change of the angular velocity caused by the torsional vibration of the engine drive system in the period from detection of the first angular velocity .omega.a to detection of the second angular velocity .omega.b. For example, if the angular velocity increased due to the torsional vibration of the engine drive system in the period from detection of the first angular velocity .omega.a to detection of the second angular velocity .omega.b, the amount of increase of the second angular velocity .omega.b with respect to the first angular velocity .omega.a will include in addition to the amount of increase of the angular velocity due to the combustion pressure the amount of increase of the angular velocity due to the torsional vibration of the engine drive system. Therefore, even if the amount of increase of the second angular velocity .omega.b due to the combustion pressure is constant when a torsional vibration occurs in the engine drive system, that is, even when there is no change in the generated torque, the generated torque calculated based on the angular velocities .omega.a, .omega.b will fluctuate and therefore there will be the problem that it will not be possible to calculate the true torque fluctuation.